Fiber reinforced plastics using inorganic fibers as reinforcing materials have excellent tensile properties, compression properties and flexural properties and are light-weight. Therefore, in certain fields, there have been begun to use them as structural materials instead of metallic materials. Particularly, among fiber reinforced plastics, those using carbon fibers as the inorganic fiber reinforcing materials have excellent light-weight properties in addition to the above properties comparable to metallic materials and, therefore, they are making rapid progress as structural materials for aircrafts and space vehicles.
Their use as structural materials is also gradually expanded to sports goods such as materials for fishing rods, golf club shafts, tennis rackets and the like. Further, at present, demand for them are expanded in the fields of light-weight sports bicycles, wheelchairs, robots and the like.
However, carbon fiber reinforced plastics are brittle against impact and liable to cause brittle fracture when they break, which results in defects such as scattering of pieces of broken plastics and the like, although they have the above excellent properties. As one reason of this, it can be mentioned that an elongation of carbon fibers is very low. Another reason of this is the nature of a material of a matrix, i.e., brittleness of epoxy resin, which is widely employed as a material for a matrix of a structural fiber reinforced plastic because of its excellent heat resistance and chemical resistance.
In order to remove the defects, recently, there have been developed improved carbon fibers which have a high strength and a high elongation, from the viewpoint of reinforcing fibers. From the viewpoint of resins for the matrix, there have been developed improved resins which have a high elongation with maintaining their inherent properties.
However, even if these improved materials are combined, required satisfactory toughness is not yet obtained and, on the other hand, there is a limit in improvement of materials.
Then, at present, development of fiber reinforced plastics is carried out predominantly by effectively utilizing already existing materials such a carbon fibers compounded (hybridized) with aramide fibers which have high impact properties.
In reinforced plastics hybridized with aramide fibers, impact properties are considerably improved in comparison with plastics reinforced with 100% of carbon fibers. However, in order to impart required toughness, it is necessary to incorporate a considerable amount of aramide fibers with carbon fibers at the sacrifice of compression properties and flexural properties of fiber reinforced plastics. Further, aramide fibers have a high water absorbing capacity at standard conditions such as 3.5% and there is such a problem that properties thereof are deteriorated with time due to water absorption under conditions of use. Furthermore, among organic fibers, aramide fibers have a very high specific gravity such as 1.45 and this should be improved because it is desired to make fiber reinforcing materials as light as possible.